The present invention relates in general to a longitudinal measuring device having a graduated measuring scale mounted on a support structure and a scanning unit for scanning the scale. More specifically, this invention relates to such measuring devices in which the scanning unit is coupled to a driving unit which is connected to the object whose position is to be measured, and in which the scanning unit is positioned against two guide surfaces of the scale and/or the support structure. This type of measuring instrument is often used in measuring the relative movements of moveable machine parts, as for example in numerically controlled machine tools.
The measuring instrument described in West German OS No. 23 49 944 utilizes a spring loaded arm to couple the scanning unit to the driving unit. This spring loaded arm is connected to the driving unit at one end and is provided with a spherical surface at the other end that enters into a recess formed in the scanning unit. In addition to performing the coupling function, this spring loaded arm also provides a positioning force which acts to press the scanning unit against two mutually perpendicular guide surfaces of the instrument.
U.S. Pat. No. 3,816,003 describes means for coupling a scanning unit to a driving unit which includes a wire which is clamped with one end to the driving unit and is provided with a bend at the other end which clamps into a recess formed in the scanning unit. As in the previously described instrument, this coupling means also acts to provide a spring force for pressing the scanning unit onto two mutually perpendicular guide surfaces of the instrument.
Measuring instruments such as these have the disadvantage that the two functions of coupling and guiding the scanning unit are performed by a single structure. Thus, the two functions are not separated from one another and an optimal coupling of the scanning unit to the driving unit, for example in the neutral phase, is generally not possible. Furthermore, when the scanning unit is pressed against two mutually perpendicular guide surfaces by the same spring arm, the positioning forces often cannot be optimally adjusted. Because a single diagonally operating spring forces presses the scanning unit onto both guide surfaces, the seating force on either of these surfaces cannot be easily or accurately set without affecting the seating force on the other surface.
In West German OS No. 2 510 219 the scanning unit is coupled to the driving unit in two mutually perpendicular planes in a flexible manner via plate springs and/or kink resistant wires. The scanning unit is guided onto two guide surfaces, which are also mutually perpendicular, by means of guide shoes which bear on the scale and rollers which bear as the support structure. The guiding forces are provided by plate springs which are installed diagonally with regard to the two guide surfaces. These plate springs are attached to the scanning unit and bear on the support structure via a roller. In this arrangement the functions of coupling and guiding the scanning unit are separated from each other; however, the use of diagonally positioned plate springs to press the scanning unit against the two mutually perpendicular guide surfaces of the scale and the support structure presents the same disadvantages that have already been mentioned.